Weapon Tuning is a new mechanic added to MW2, but the feature is currently not working.
The option to tune your weapon and personalize attachments has been added so that players may create weapons that are more suited to their play style.
However, players that have been working hard to unlock and use weapon tuning will discover the feature is not available.
Infinity Ward has already commented on this issue, giving us the reasons why weapon tuning is not working in MW2.

Why Can’t I Tune Weapons in MW2?
Infinity Ward has disabled weapon tuning in Modern Warfare 2 due to reports that tuning 5 attachments on a loadout was causing the game to crash for players.
To even use a previously tuned loadout, you will need to unequip and re-equip any tuned attachments. This will give you the standard attachments whilst tuning is disabled.
MW2 players have been reporting that their game has crashed while selecting certain loadouts. Weapon tuning 5 attachments is the reason for these crashes.
Twitter user @ItzCrusty_ has demonstrated what happens when you select a loadout with 5 tuned attachments.
Why Does My Game Crash When I Choose Loadout in MW2?
If you are not able to select and use your loadout in MW2, it is likely because you have 5 tuned attachments on your loadout.
With Infinity Ward disabling weapon tuning, having 5 attachments tuned is still the main cause of the game crashing.
To stop Modern Warfare 2 from crashing, you will need to unequip and reequip your current attachments. This process works because you’ll be reequipping untuned attachments.

When Will I Be Able To Access Weapon Tuning in MW2?
Infinity Ward has re-enabled weapon tuning in the new MW2 version 1.08 update on November 4.
This means that you can now tune your attachments, and the game will no longer crash when you tune 5 attachments on a loadout.
MW2’s launch has not been smooth so far! Players were reporting that they couldn’t apply gold camo to their weapons, however, a fix for all gold camo bugs has been found.
A fast way to level up weapons has been discovered in Modern Warfare 2! You can use this method to level up your weapon, so you can tune it once it has been brought back.